This invention relates to the manufacture of substrates, such as paper, having an electroconductive coating useful when the paper is employed as a recording element in electrographic printing or image reproducing processes. More particularly it relates to a substrate having on at least a portion of its surface the reaction products from the interaction between monomeric epichlorohydrin and a tertiary amine as the electroconductive component in the coating and to the process for producing coated substrate product.
It is a requirement in many electrographic processes that the sheet or substrate receiving the image have a surface which will accept an electrostatic charge and hold it for a few minutes in darkness. Exposure to light makes the illuminated areas electroconductive and causes dissipation of the charge from the exposed surface. If an image to be reproduced is projected by light onto the surface of the sheet, the charge dissipates from the bright parts of the image but is retained in the dark parts. Suitable toners are attracted electrostatically to the charged regions and fixed, either by heat, solvent evaporation or other means to give a permanent reproduction of the image.
Imparting electrical conductivity to paper is not difficult provided that the paper is used at relatively high humidities, say 50% or more. Simple, water soluble inorganic salts which have a degree of hygroscopicity suffice. Such salts, including ammonium chloride, calcium chloride, etc., were widely used in the past. However, at very high humidities the paper containing these salts tends to become damp and curl up and exhibit all of the aesthetically objectionable properties of wet paper. On the other hand, at low humidities, the paper tends to dry out and conductivity is impaired. The result is a poor reproduction wherein toner is attracted to the whole sheet surface to a greater or lesser extent rather than just to areas which had not been illuminated.
It has become conventional more recently to employ polymers containing pendant ionic groups, such as trimethylammonium, sulfonium, phosphonium, carboxylate, and so on, all in the form of their salts with simple counterions such as chloride, sodium, potassium or ammonium to give improved performance in the low humidity regions. The improved performance is sufficient that such polymers have largely displaced the simple salts as electroconductive paper components.
Illustrative of coated paper products employing polymers containing pendant ionic groups are Silvernail et al. U.S. Pat. No. 3,011,918 employing polymerized vinylbenzyl quaternary ammonium compounds as the paper coating. Poot et al. U.S. Pat. No. 3,625,684 and Jursich et al. U.S. Pat. No. 3,640,766 employ polyepichlorohydrins quaternized by reaction with a tertiary amine, such as trimethylamine.
The quaternized polymers of the foregoing types are difficult to produce and, consequently, expensive. A monomer must be polymerized in a controlled manner to give a water soluble product of viscosity suitable for use in a coating mixture. Such control is, at times, difficult. The polymer must then be subjected to a suitable chemical reaction wherein there is attached an amino, sulfonium, phosphonium, carboxylic group, etc., to the polymer. Quite often the steps are reversed; that is, the pendant group is attached first and the intermediate is then polymerized. This procedure is just as difficult.
The quaternary ammonium group is now the preferred pendant group of those experienced in the art. Apparently, this is because of superior conductivity (M. F. Hoover and H. E. Carr, TAPPI, 51, 552-559, 1968). One such polymer is quaternized polyepichlorohydrin of said U.S. Pat. Nos. 3,640,766 and 3,625,684. Epichlorohydrin must first be polymerized using the highly pyrophoric and dangerous triisobutyl aluminum as catalyst. Such a method is described in U.S. Pat. No. 3,640,766. As triisobutyl aluminum is extremely reactive to water, traces of water must first be removed by distillation from the epichlorohydrin. Polymerization requires temperatures of around 100.degree. C. After polymerization is complete, excess monomer is removed by steam distillation and the polyepichlorhydrin is quaternized by treatment with trimethylamine. Quaternization is a long, slow reaction requiring considerable expenditure of energy for heating. Times as long as 16 or 17 hours at 100.degree. C. are mentioned in the patent.
Similar objections to a greater or lesser extent apply to the preparation of other polymers with pendant trimethylammonium groups.
In contrast to this, products and processes of the present invention involve reaction temperatures not higher than 100.degree. C., for times as short as a minute or two, aqueous amine solution is used and so the monomeric epichlorohydrin need not be freed of traces of moisture, and a single reaction step, without subsequent distillation or other purification steps, is employed. In common with the quaternized polyepichlorohydrins, we, too, find it desirable to remove traces of amine by neutralization with hydrochloric acid or by quaternization with methyl chloride. This is desirable to remove objectionable amine odor from the product.
It is, accordingly, an object of the present invention to provide a novel electroconductive surface applied to a substrate, such as paper, suitable for electrographic image reproduction.
It is also an object of the present invention to provide a novel process for producing an electroconductive surface on a paper substrate which process eliminates the disadvantages of prior art processes for producing such products.
Other objects will be apparent to those skilled in the art from the present description.